Multi-stage hydrocracking process



March 8, 1966 C; P REEG ET AL 3,239,447

MULTI-STAGE HYDROCRACKING PROCESS Filed May 7, 1962 nited States Patent O 3,239,447 MULTI-STAGE HYDRCRACKING PRCESS Cloyd P. Reeg, Orange, Bernal Peralta, Anaheim, and

John H. Duir, Fullerton, Calif., assignors to Union Oil Company of California, Los Angeles, Calif., a corporation of California Filed May 7, 1962, Ser. No. 192,691 Claims. (Cl. 208-59) This invention relates to new techniques for the operation of multi-stage hydrocracking processes, so as to produce therefrom both a relatively high-octane gasoline product, and a relatively non-aromatic, low-freezing-point jet fuel product. In its simplest modification, the invention embraces two stages of hydrocracking, the first stage being operated under optimum conditions for producing relatively high-octane gasoline, and the second stage being operated under optimum conditions for the production of high-quality jet fuels. The optimum conditions referred to for each stage involve a .proper selection of ternperatures, pressures, catalys-ts, feedstocks, and most importantly to this particular invention, an optimum selection of specific recycle fractions for each of the hydrocracking stages.

Briefly stated, the invention comprises the following steps: (l) a gas oil feedstock is subjected to a first stage of hydrocracking at relatively high temperatures, permissibly in the presence of contaminants such as hydrogen sulfide and/or ammonia; (2) the efiiuent from the first-stage hydrocracking is fractionated to recover Aa relatively high-octane gasoline product, an intermediate-boiling-range light gas oil, and a high-boiling bottoms fraction; (3) the light gas oil is recycled to the first hydrocracking stage; (4) the bottoms fraction is sent to a second hydrocracking stage which operates at relatively lower temperatures and substantially in the absence of nitrogen and sulfur compounds; (5) the effluent from the second hydrocracking stage is fractionated to recover a smal-1 -amount of light gasoline boiling up to about 350 F., a desired jet fuel product praction, and a heavy gas oil bottoms fraction; and (6) the heavy gas oil ybottoms fraction is either util-ized as diesel fuel or recycled to the second hydrocracking stage. It is found that by operating in this manner, with recycle of a light gas oil to the first stage, and only heavier -gas oils to the second stage, higher' quality jet fuels, and larger quantities of high-octane gasoline are obtained, as compared to a case where there is no re-cycle of unconverted oil to the first hydrocracking stage, i.e., a case where all of the unconverted oil from the first stage boiling above the gasoline range is treated exclusively in the second stage.

According to a preferred modification of the invention, where the initial feed contains substantial amounts of nitrogen and/or sulfur compounds, the first hydrocracking stage is preceded by an integral hydrofining treatment. In this pretreatment step, the feed is contacted with a hydrofining catalyst under conditions of 4temperature and pressure similar to those prevailing in the first hydrocracking stage, to thereby convert organic sulfur and nitrogen compounds to hydrogen sulfide and ammonia. The total effluent from the hydrofining step is then transferred directly to the first hydrocracking stage without intervening treatment for the removal of ammonia and/or hydrogen sulfide. It has been found that hydrogen sulfide in the first hydrocracking stage is beneficial from the standpoint of improving the octane number of the gas-oline produced therein.

In copending application Serial No. 134,935, filed August 30, 1961, now U.S. Patent No. 3,132,087, it is shown that in two-sta-ge hydrocracking processes, where the `unconverted oil from the first stage is treated in the second stage, a higher octane gasoline is produced in the ice first stage than in the second stage, and that the jet fuel fraction produced from the second stage is superior to the jet fuel fraction from the first stage. The present invention is based upon our discovery that larger quantities of high octane gasoline are recoverable from the first stage, and still higher quality jet fuels are recoverable from the second stage, if the light gas oil product fraction from the first stage is recycled back to that stage instead of being transferred to the second stage. The explanation for these improved results is not entirely clear, but a partial explanation may be found in the following factors:

Firstly, in order to produce low-freezing-point jet fuels it is necessary to reduce the content of normal parafiins and aromatics, and increase the isoparaflin and naphthene content. Virgin jet fuel fractions are relatively rich in normal parafiins, while jet fuel fractions produced by the hydrocracking of oils boiling above the jet fuel range, i.e., completely synthetic jet fuels, are relatively richer in isoparafins as compared to nor-mal paraffins. As a result of eliminating the light gas oil fraction from the feed to the second stage, all of the jet fuel produced therein will be synthetic, i.e., produced from hydrocarbons boiling Iabove the jet fuel range, and will consequently be richer in isoparaffins.

Secondly, by recycling the 'light gas oil product fraction from the first hydrocracking stage back to that stage, the total feed thereto, i.e., fresh feed plus recycle, becomes relatively -rich in hydrocarbons boiling between about 400 and 550 F., and relatively lean in hydrocarbons boiling Iabove 550 F. The hydrocracking conditions in the first stage can then be adjusted more selectively to minimize hydrogenation and thus increase aromaticity of the gasoline produced. This entails using somewhat lower pressures and/or higher temperatures, factors which normally tend to cause a more rapid deactivation of the catalyst. But since deactivation is due largely to the presence of heavy aromatic hydrocarbons boiling above 550 F., and since the concentration of these hydrocarbons is reduced by the recycle dilution, their deactivating effect is also minimized. Consequently, it will be seen that, as a result of the recycle to the first stage, each stage of the hydrocracking can be adjusted more selectively, so as to produce even more highly aromatic gasolines from the first stage, and more highly naphthenic and isoparafiinic jet fuels from the second stage.

From the foregoing it will be apparent that the principal purpose of this invention is to provide an integrated hydrocracking process designed mainly for the production of high-octane gasoline, but which can also produce a high-quality, low-freezing-point jet fuel, boiling for example in the 350-550 F. range. A specific object is to provide a hydrocracking process which will produce jet fuels having a freezing point below about -60 F. Other objects will be apparent from the more detailed description which follows.

The invention may perhaps be more readily understood with reference to the attached drawing, which is a flowsheet illustrating a preferred form of the invention. The initial feedstock is brought in Via line 2, mixed with with recycle light gas oil from line 3 (if desired), and with recycle and makeup hydrogen from line 4. The mixture is then preheated to incipient hydrofining temperature in heater 6, and passed directly into hydrofiner 8, where hydrofining proceeds under substantially conventional conditions. Suitable hydrofining catalysts include for example mixtures of the oxides and/or sulfides of cobalt and molybdenum, or of nickel and tungsten, preferably supported on a carrier such as alumina, or alumina containing a small amount of coprecipitated silica gel. Other suitable catalysts include in general the oxides and/or sulfdes of the Group VIB and/or Group VIII metals, preferably supported on adsorbent oxide carriers such as alumina, silica, titania, and the like. The hydroning operation may be conducted either adiabatically or isothermally, and under the following general conditions:

Hydrofining conditions The above conditions are suitably adjusted so as to reduce the organic nitrogen content of the feed to below Vabout 25 parts per million, and preferably below about parts per million.

The total hydroiined product from hydroner 8 is taken olf in line 10 and transferred via heat exchanger 12 to first-stage hydrocracker 14, without intervening condensation or separation of products. Heat exchanger 12 is for the purpose of suitably adjusting the temperature of feed to hydrocracker 14; this may Irequire either cooling or heating, depending uponthe respective hydroiining and hydrocracking temperatures employed. Inasmuch las firststage hydrocracker 14 and hydroner 8 a-re preferably operated at essentially the same pressure, .it is entirely feasible to enclose both contact-ing zones within a single reactor, using appropriate temperature control means.

The process conditions in hydrocracker 14 are suitablyadjusted so as to provide about 70% conversion to 400 F. end-point gasoline and lighter products per pass, while at the same time permitting relatively long ru-ns between regenerations, i.e., from about 2 to 8 months. For these purposes, as well :as for maintaining product quality within the 85-100 -octane range l(rF-l-l-S ml. TEL), it will be understood that tempera-Y tures in the high range will be used in connection with pressures in the high range, while the lower temperatures will normally be used in conjunctionwith lower pressures. The range of operative conditions contemplated for reactor 14 lare as follows:

F irai-stage hydrocrackng conditions Operative Preferred Temperature, F 500-850 60G-800 Pressure, p.s.i.g. 400-3, 000 50G-2, 000 L v./v./hr. l 0. 5-10 l-5 Hz/oil ratio, s.c.f./ 500-15, 000 1, OOO-10, U00

ing column 38.

In fractionating column 38, the first-stage hydrocrackling efuent is split into three fractions, as follows:

(1) A G50-400 F. end-point gasoline fraction which is taken overhead via line 40. This gasoline will have an octane number (F-lr-i-S ml. TEL) lof about 85-100, and may in many cases be used directly as blending stock -for high octane gasolines. In `other cases, it may be de- Operative Preferred 60G-850 650-750 500-3, 000 BOO-2, (1)00 v -o 500-15, O00 1, OOO-10, 000

-arator 32 are lthen transferred via line 36 to fractionatsirable to subject it toa mild reforming operation to gain stage hydrocracker.

The bottoms fraction in line 461is then blended with t second-stage recycle oil fromline 48, and this Vcombined point of the side-cut should preferablyV fall within about $20 F. of the desired end-boiling-point of the yjet fuel product to be produced from the second stage hydrov cracker. Since most commercial grades of jet fuels have an end-boiling-point which lies between about 500 and 550 F., the end-boiling-point of the side-cut inline 42 will preferably also fall within thatrange'.- In this man-:D

ner, substantially all of the jet fuel synthesized in the second-stage hydrocracker will .be derived from hydrocarbons boiling above the end-boiling-point 'of the desired jet fuel product. plated that any selected fraction vof the first-stage product,

which fraction: boils within therboiling frange of the desired jet yfuel product, may be recycled to the first stage.

(3) A bottoms fraction from column 38 is recovered via line 46, and comprises the remaining hydrocarbons boiling above the end-boiling-point of the side-cut in line 42.1 Thus, the initial boiling pointfof the bottoms fraction will normally be between about 500 and 550 F., this constituting the primary feedstock to the second,-v

feed in line 50 is then mixed with recycle and makeup hydrogen from line 52, preheated to incipient hydrocracking temperatures in heater 54-,iandv passed intosecondstage hydrocracker 56. This feedstock differs considerably from-the feed to thefirst stage hydrocracker in that it is substantially free of nitrogen compounds and has a higher initial boiling point. Itis also substantially free of sulfur compounds, and hence the second-stage hydrocracker may be operated entirely sweet if desired by simply removing .hydrogen sulfide from the recycle gas to that stage. second stage sweet, inorder to maintain the hydrogenating component'on the catalyst in a metallic state, the metallic state normally being the most active Iform for hydrogenation. Moreover, due to the.V substantial absence of nitrogen compounds, and the objective of maximizing jet fuel production rather than gasoline, lower temperatures can be maintained in hydrocracker 56. The process conditions in hydrocracker 56 are preferably adjusted so as to provide from 40-80% conversion to jet fuel and lighter products per pass. To achieve this objective, the hydrocracking conditions should be suitably correlated within the following general ranges:

Second-stage hydrocracking conditions At the conversion levels and conditions prescribed for the second-stage hydrocracker, the run `length between catalyst regenerations can be adjusted to coincide substantially with the run length inreactor 14, e.g., between about 2 and 8 months. In extended `runs such as these',v

it is normally desirable to maintain substantially constant conversion in each stage by incrementally raising the temperature as the activity of the catalyst declines. The? l rate of catalyst activity decline in rsealcktopsf14a'd 56 under thek PrescribedV condition/sgi uc" that constant Broadly however, it is contemt Ordinarily, it is preferred to operate the conversion in both re'actors can be obtained by raising the respective temperatures between about 0.1 and 3 F. per day, on the average. The average temperature in hydrocracker 56 will normally be about 50-200 F. lower than the average temperature in hydrocracker 14.

The total etiiuent from hydrocracker 56 is withdrawn via line 58, condensed in cooler 60 and transferred to high pressure separator 62, from which recycle hydrogen is Withdrawn via line 64. The condensed hydrocarbons in separator 62 are then flashed via line 66 into low pressure separator 68, from which flash gases are exhausted via line 70. The liquid hydrocarbon product in separator 68 is withdrawn via line 72 and transferred to second-stage product fractionation column 74, wherein it is fractionated into gasoline, jet fuel and bottoms recycle fractions, as in column 38. A small proportion of light gasoline blending stock is withdrawn as overhead via line 76, and a jet fuel side-cut product fraction via line 78. This jet fuel product will normally have an initial boiling point between about 300 and 400 F., an endboiling-point between about 500 and 550 F., and a freezing point between about 65 and 100 F. The bottoms from column 74 is taken off in line 80, and may be withdrawn from the system Via line 82 and utilized as diesel fuel, or it may be recycled via line 48 to second-stage hydrocracker 56 as previously noted. It will be understood that the choice of these alternatives depends largely upon the desired refinery balance and market demands.

The hydrocracking catalysts to be employed in the hydrocracking units described above may consist of any desired combination of a refractory cracking .base with a suitable hydrogenating component. Suitable cracking bases include for example mixtures of two or more refractory oxides such as silica-alumina, silica-magnesia, silica-zirconia, alumina-boria, silica-titania, silica-Zirconiatitania, acid treated clays and the like. Acidic metal phosphates such as aluminum phosphate may also be used. The preferred cracking bases comprise composites of silica and alumina containing about 50-90% silica; coprecipitated composites of silica, titania, and zirconia containing between 5% and 75% of each component; partially dehydrated, zeolitic, crystalline molecular sieves, e.g., of the X or Y" crystal types, having relatively uniform pore diameters of about 8 to 14 Angstroms, and comprising silica, alumina and one or more exchangeable zeolitic cations. Any of these lcracking bases may be further promoted by the addition of small amounts, e.g., 1 to 10% by weight, of halogen or halides such as fluorine, boron trifluoride or silicon tetrauoride.

The molecular sieve type cracking bases, when compounded with a hydrogenating metal, are particularly useful for hydrocracking at relatively low temperatures of 40G-700 F., and relatively low pressures of SOO-1,500 p.s.i.g. It is preferred to employ molecular sieves having a relatively high SiO2/Al203 ratio, e.g., between about 2.5 and 6.0. The most active forms are those wherein the exchangeable zeolitic cations are hydrogen and/or a divalent metal such as magnesium, calcium or zinc. In particular, the Y molecular sieves, wherein the SiO2/Al203 ratio is about 5, are preferred, either in their hydrogen (or decationized) form, or a divalent metal form, preferably magnesium. Normally, such molecular sieves are prepared first in the sodium or potassium form, and the monovalent metal is ion-exchanged out with a divalent metal, or where the hydrogen form is desired, with an ammonium salt followed by heating to decompose the zeolitic ammonium ion and leave a hydrogen ion. It is not necessary to exchange out all of the monovalent metal; the final compositions may contain up to about 6% by weight of NaO, or equivalent amounts of other monovalent metals. Catalysts of this nature are more particularly described in Belgian Patents Nos. 598,582, 598,- 682, 598,683 and 598,686.

As in the case of the X molecular sieves, the Y sieves also contain pores of relatively uniform diameter in the individual crystals. In the case of X sieves, the pore diameters may range between about 6 and 14 A., depending upon the metal ions present, and this is likewise the case in the Y sieves, although the latter usually are found to have crystal pores of about 9 to 10 A. in diameter.

The foregoing cracking bases are compounded, as by impregnation, with from about 0.5 to 25% (based on free metal) of a Group VIB or Group VIII metal promoter, e.g., an oxide or sulfide of chromium, tungsten, cobalt, nickel, or the corresponding free metals, or any combination thereof. Alternatively, even smaller proportions, between about 0.05% and 2% of the metals platinum, palladium, rhodium or iridium may be ernployed. The oxides and suldes of other transitional metals may also be used, but to less advantage than the foregoing.

In the case of the zeolitic type cracking bases, it is preferable to distribute the hydrogenating metal thereon by ion exchange. This can be accomplished by digesting the zeolite with an aqueous solution of a suitable compound of the desired hydrogenating metal wherein the metal is present in a cationic form, and then reducing to form the free metal, as described for example in Belgian Patent No. 598,686.

The feedstocks which may be treated herein include in general any mineral oil fraction having an initial boiling point above the conventional gasoline range, i.e., above about 400 F., and having an end-boiling-point above the end-boiling-point of the desired jet fuel product, and up to about 1,000 F. This includes straight-run gas oils, coker distillate gas oils, deasphalted crude oils, cycle oils derived from catalytic or thermal cracking operations and the like. These fractions may be derived from petroleum crude oils, shale oils, tar sand oils, coal hydrogenation products and the like. Specifically, it is preferred to employ feedstocks boiling between about 400 and 900 F., having an API gravity of about 15 to 35, containing at least about 20% by volume of acid-soluble components (aromatics-i-olefins), and at least about 20% by volume of hydrocarbons boiling above 650 F. Such oils may also contain from about 0.1% to 5% of sulfur and from about 0.001% to 2% by weight of nitrogen.

The following example is cited to illustrate the process as above-described in connection with the drawing, but should not be construed as limiting in scope:

EXAMPLE In this example, a blend of delayed Coker distillate and thermally cracked gas oils derived from California crude oils is utilized as feed. The treatment comprises an initial catalytic hydrofining, with the total hydrofining effluent passing to the first stage of hydrocracking. The firststage hydrocracking effluent is water-Washed and fractionated to recover a 400 F. end-point gasoline fraction, a 40S-510 F. light gas oil fraction which is recycled to the first stage, and a bottoms fraction which is fed to the second stage of hydrocracking. The effluent from the second hydrocracking stage is condensed and separately fractionated to recover a 350 F. end-point light gasoline fraction, a S60-510 F. jet fuel fraction and a bottoms fraction, the latter being recycled to the second stage of hydrocracking. The significant conditions, specifications and results of the process are as follows:

Initial feedstock:

Boiling range, F 400-850 Acid-solubles, vol. percent 65 Gravity, API 21 Nitrogen, wt. percent 0.4

Sulfur, wt. percent 2 Hydr-ofining conditions:

Catalylst: 3% CoO, 15% M003 on 5% SiO2 A1203 carrier; catalyst presulfided. Temperature, avg. bed, F 705 Pressure, p.s.i.g 1,500

7 LHSV 1.0 Hz/oil ratio, s.c.f./:b 8,000 First-stage hydrocracking conditions:

Catalyst: Magnesium Y molecular sieve loaded with 0.5% Pd (Linde hydrocracking catalyst MB 5382).

Temperature, avg. bed, F. 730

Pressure, p.s.i.g 1,500

LHSV 1.5

HZ/oil ratio, s.c.f./b. 8,000

Conversion per pass to 400 F. end-point gasoline and lighter, vol. percent 45 Second-stage hydrocracking conditions:

Catalyst: Hydrogen (decationized) Y molecular sieve loaded with 0.5% Pd (Linde isomerization catalyst MB 5390).

Temperature, avg. bed, F. 550

Pressure, p.s.i.g 1,500

LHSV 1.5

Hg/oil ratio, s.c.f./b. 8,000

Conversion per pass to 360-510 F. jet fuel lighter, vol. percent 65 Approximate material balances (per 100 bbls. fresh feed) Ultimate liquid products: Barrels TOtal C4*C5 First-stage C7-400 F. gasoline 40 Second-stage C7-350 F. gasoline 25 Second-stage 360-510 F. jet fuel 35 The leaded octane number of the rst-stage C7-400 F. gasoline is about 94-98, and the freezing point of the second-stage jet fuel is about -75 F. If'the entire firststage product fraction boiling above 400 F. were sent to the second stage, the freezing point of the 360-510 F. jet fuel product would be about 50 F.

Results analogous to those indicated in the foregoing eX- ample are obtained when other hydrocracking catalysts and conditions, other feedstocks and Iother hydroning conditions wit-hin the broad purview of the above disclosure are employed. It is hence not intended to li-mit the invention t-o the details of the example or the drawing, but `only broadly as defined in the following claims.

We claim:

1. An integrated process for the manufacture of highoctane gasoline and high-quality jet fuel by hydrocracking which comprises: (l) subjecting a gas oil feedstock containing hydrocarbon components boiling above the end-point of the desired jet fuel product, and also containing organic sulfur and/or nitrogen compounds, to catalytic hydroning to eifect decomposition of said sulfur and nitrogen compounds; (2) subjecting eiuent from said hydrofining, without intervening treatment for the removal of ammonia and hydrogen sulde, to a rst stage compounds; (4) fractionating the hydrocarbon product from said rst-stage hydrocracking to recover (a)k a gasoline product fraction having an end-boiling-poiut above about 350 F., (b) a light gas oil fraction boiling above said gasoline product fraction and yhaving Ian :end-boilingpoint between about 500 and 550 F., and (c) a heavy gas oil boiling above the end-boiling-point of said light* gas oil fraction and above the end-boiling-point of the desired jet fuel product; (5) recycling saidV light gas oil to said rst hydrocracking-stage; y(6) subjecting said heavy gas oil to a second stage of catalytic hydrocracking in the presence of a vGroup VIII metal-molecular sieve zeolite type hydrocracking catalyst, rand atfa temperature `which is (a) between about 400- and' 750 F., and (b) lower than the temperature employed in said tiret-stage hydro. cracking, and at a space velocity adjusted to give about 40 to 80% conversion to jet fuel and lighter. products per pass; (7) fractionating the eflluent from said second-stage hydrocracking to recover a light gasoline fraction, a jet fuel product fraction having an initial boiling pointbe-V tween about 300 and .400 F., an rend-boiling point betweenabout 500 vand 550 F., and having a freezing point below about -60 F. which is withdrawn Vfrom the process, and a 'heavy bottoms fraction; and (8)V re- 1 cycling at least a portion of said heavy bottoms fraction to said second hydrocracking stage.

2. A process as dened in claim 1 wherein said rst stage of hydrocracking i-s conducted at a temperature 'between about 50 andv200 F. higher than the temperature in said second-stage hydrocracking.

3. A process -as dened in claim 1 wherein said secondstage hydrocracking iscarried out substantially in the absence of sulfurand nitrogen compounds.

4. A process-as dened in claim 1 wherein the hydrocracking in each of said stages is car\ried out` ata pressure 'below about 3,000 p.s.i.\g.

5. A process as dened inclam 1 whereina hydrocracking catalyst comprising a dehydrated, zeolitic molecular sieve of the fY crystal type containing zeolitic cat-r ions fromthe group consisting of hydrogen and divalent metals, and promoted withA a GroupVIII noble metal hydrogenating component, is used in each .of said hydrocracking stages.

PAUL .M COUGHLAN, Primary Examiner.

ALPHONSO D SULLIVAN, Examiner.v 

1. AN INTEGRATED PROCESS FOR THE MANUFACTURE OF HIGHOCTANCE GASOLINE AND HIGH-QUALITY JET FUEL BY HYDROCRACKING WHICH COMPRISES: (1) SUBJECTING A GAS OIL FEEDSTOCK CONTAINING HYDROCARBON COMPONENTS BOILING ABOVE THE END-POINT OF THE DESIRED JET FUEL PRODUCT, AND ALSO CONTAINING ORGANIC SULFUR AND/OR NITROGEN COMPOUNDS, TO CATALYTIC HYDROFINING TO EFFECT DECOMPOSITION OF SAID SULFUR AND NITROGEN COMPOUNDS; (2) SUBJECTING EFFLUENT FROM SAID HYDROFINING, WITHOUT INTERVENING TREATMENT FOR THE REMOVAL OF AMMONIA AND HYDROGEN SULFIDE, TO A FIRST STAGE OF CATALYTIC HYDROCRACKING AT A TEMPERATURE BETWEEN ABOUT 500* AND 850*F. AND A PRESSURE BETWEEN ABOUT 400 AND 3,000 P.S.I.G., SAID TEMPERATURE BEIN ADJUSTED TO GIVE A 20-70% CONVERSION PER PASS TO 400*F. END-POINT PRODUCTS; (3) TREATING THE EFFLUENT FROM SAID FIRST-STAGE HYDROCRACKING TO SEPARATE OUT WATER-SOLUBLE SULFUR ANDNITROGEN COMPOUNDS; (4) FRACTIONATING THE HYDROCARBON PRODUCT FROM SAID FIRST-STAGE HYDROCRACKING TO RECOVER (A) A GASOLINE PRODUCT FRACTION HAVING AN END-BOILING-POINT ABOVE ABOUT 350*F., (B) A LIGHT GAS OIL FRACTION BOILING ABOVE SAID GASOLINE PRODUCT FRACTION AND HAVING AN END-BOILINGPOINT BETWEEN ABOUT 500* AND 550*F., AND (C) A HEAVY GAS OIL BOILING ABOVE THE END-BOILING-POINT OF SAID LIGHT GAS OIL FRACTION AND ABOVE THE END-BOILING-POINT OF THE DESIRED JET FUEL PRODUCT; (5) RECYCLING SAID LIGHT GAS OIL TO SAID FIRST HYDROCRACKING STAGE; (6) SUBJECTING SAID HEAVY GAS OIL TO A SECOND STAGE OF CATALYTIC HYDROCRACKING IN THE PRESENCE OF A GROUP VIII METAL-MOLECULAR SIEVE ZEOLITE TYPE HYDROCRACKING CATALYST, AND AT A TEMPERATURE WHICH IS (A) BETWEEN ABOUT 400* AND 750*F., AND (B) LOWER THAN THE TEMPERATURE EMPLOYED IN SAID FIRST-STAGE HYDROCRACKING, AND AT A SPACE VELOCITY ADJUSTED TO GIVE ABOUT 40 TO 80% CONVERSION TO JET FUEL AND LIGHTER PRODUCTS PER PASS; (7) FRACTIONATING THE EFFLUENT FROMSAID SECOND-STAGE HYDROCRACKING TO RECOVER A LIGHT GASOLINE FRACTION, A JET FUEL PRODUCT FRACTION HAVING AN INITIAL BOILING POINT BETWEEN ABOUT 600* AND 400*F., AN END-BOILING POINT BETWEEN ABOUT 50* AND 550*F., AND HAVING A FREEZING POINT BELOW ABOUT -60*F. WHICH IS WITHDRAWN FROM THE PROCESS, AND A HEAVY BOTTOMS FRACTION; AND (8) RECYCLING AT LEAST A PORTION OF SAID HEAVY BOTTOMS FRACTION TO SAID SECOND HYDROCRACKING STAGE. 